Tin coated steel article



United States Patent 3,326,646 TIN COATED STEEL ARTICLE George K. Notman, Pittsburgh, Pa., assignor to Jones &

Laughlin Steel Corporation, Pittsburgh, Pa., 2 corporation of Pennsylvania No Drawing. Filed Feb. 3, 1965, Ser. No. 430,202

2 Claims. '(Cl. 29-1835) This invention relates to the production of electrolytic tinplate of improved corrosion resistance. It is more particularly concerned with such electrolytic tinplate having a modified tin-iron alloy layer, and with a process for producing such tinplate.

Most tinplate now being produced is provided with its tin coating by an electrotinning process. Such a process permits plating of a relatively thin tin coating evenly distributed over the base. Because the thickness of this tin coating on most tinplate commercially used is a small fraction of an inch, the thickness of the tin coating is more conveniently expressed in terms of its weight in pounds per base box of tinplate. The term base box is a measure of area or surface, and amounts to 31,360 square inches. Large amounts of electrolytic tinplate are made with coating weights of /z or pound of tin per base box of tinplate.

Tinplate used for food packs must be carefully tested to determine its resistance to corrosion by its contents. Heretofore, the determination of corrosion resistance of tinplate has been a rather tedious process involving packing the food product in question in the tinplate to be tested and examining the condition of the coating from time to time. In recent years, it has been found that the corrosion resistance of tinplate used for food packs can be gauged quite well by a galvanic test known as the alloy-tin couple test. The test consists of stripping the tin from a sample of tinplate leaving the base covered only by the tin-iron alloy layer, and measuring the current density developed by a galvanic couple comprising a pure tin electrode and the sample immersed in grapefruit juice containing 100 ppm. of soluble stannous tin at a temperature of 79 F. The current density after 20 hours is measured in microarnperes per square centimeter, and the figures so obtained are referred to as ATC values. Low ATC values indicate good corrosion resistance. The ATC test is described in the paper The Alloy-Tin Couple Test-A new Research Tool by G. G. Kamm, A. R. Willey, R. E. Reese and I. R. Krickl, published in Corrosion, volume 17, February 1961, pages 106112.

Commercial electrolytic tinplate as produced under varying conditions and with different weights of coating has ATC values ranging from as low as 0.01 up to perhaps 0.5. While the lowest possible ATC value is, of course, desirable for any weight of tinplate, ATC figures around 0.1 are commonly obtained with commercial tinplate having thin tin coatings.

Tin may be electro-deposited upon a steel base using either an alkaline or an acid electrolyte. An alkaline process is disclosed in U.S. Patent 2,424,472., issued to F. A. Lowenheim et al., on July 22, 1947. Alkaline processes have the disadvantage that they are considerably slower than acid processes, and commercial electro-tinning lines, therefore, usually employ acid electrolytes. One such electrolyte commonly used is that of U.S. Patent 2,407,579, issued to E. W. Schweikher on Sept. 10, 1946. The process using the electrolyte of that patent is commonly known as the halogen process.

It is an object of my invention to provide electro-tinned strip of improved corrosion resistance. It is another object to provide such tinplate by modification of the tin-iron alloy layer. It is another object to provide such tinplate by adding nickel to the tin-iron alloy layer. It is still another object to provide such tinplate by pre-plating the base with a tin-nickel alloy. Other objects of my invention will appear in the course of description whereof which follows.

Those skilled in the tin-plate art know that the corrosion resistance of electrolytic tinplate is influenced by the tiniron alloy layer which is formed at the tin-iron interface when the electroplated matte tin coating is brightened by reflowing. I have found that the corrosion resistance of such tin plate is improved if the tin-iron alloy is modified to include nickel. The mechanism which brings about this improvement is not known to me. I have discovered, however, that the presence of the nickel changes the physical shape of the crystals of the alloy layer so that they seem to'provide better coverage of the base metal. In an embodiment of my invention presently preferred by me, I pre-plate the steel base with a thin coating of a tin-nickel alloy. I then over-plate this coated base with tin in the conventional way to bring the coating weight of the tinplate to its desired value, and heat the doublyplated base in the conventional way to a temperature above the melting point of tin so as to flow-brighten its tin surface.

The art of providing base metals with electro-plated coatings of tin-nickel alloy is well developed. This alloy deposits in a bright form, is resistant to tarnishing, is harder and more durable than either tin or nickel, and has a faintly pinkish tint. For these reasons, it has been employed for decorative purposes. Unfortunately, these coatings are rather brittle and tend to flake off if the base metal is deformed. Coatings of this type and the process of their deposition are described in the publication, Electro-Plated Tin-Nickel Alloy, of the Tin Research Institute, Fraser Road, Greenford, Middlesex, England, issued March 1955, and revised October 1957. The electrolyte there disclosed has the following composition:

SnCl -2H O gm. per liter 50 NiCl -6H O do 250 NH F HF do 40 NH;.; 35% Sp. gr. 880 ml. per liter 35 I find it satisfactory to pre-plate my steel base by the process and using the electrolyte described in that publication, except that I do not provide the steel base with a copper undercoat as is there recommended. I pre-plate my steel base in this way with a tin-nickel alloy coating of about 1 micro-inch thickness. The elfect of alloy layer thickness will be mentioned hereinafter. The preplated base is then over-plated with tin by any conventional electrolytic process to the desired coating weight, which may range from lb. to 1 lb. per base box. I prefer to carry out this over-plating by the conventional halogen process. The tin coating is then flow-brightened by rapidly heating the tinplate to a temperature above the fusion point of tin and quenching it in the usual way. This heating causes the tin-nickel preplate to alloy with the iron of the base metal at the interface so as to form there a ternary alloy of iron, tin and nickel. Tinplate so preplated with a tin-nickel alloy coating having a thickness of 1 micro-inch and then over-plated with tin in the amount of 4 lb. per base box and flow-brightened displayed ATC values of about .011. Control tinplate made the same way, but without the tin-nickel preplate gave ATC values of .034 to .045. A coating of tin or tin-nickel alloy 1 micro-inch thick amounts to .0177 lb. per base box. The extra thickness it provides is, there-fore, of no consequence, and the greatly improved ATC values 3 of the preplated tinplate can only be attributed to the change in the tin-iron alloy layer resulting from the incorporation of nickel.

My invention is also successfully practiced with tinplate preplated with tin-nickel from an electrolyte of the following composition:

Gm. per liter snci 36.0 NaF 33.8 NaHF 16.9 NaCl 35.0 NiCl '6H O 225.0 pH, 2.3.

The over-plate and brightening are carried out as above described. The alloy deposited from this electrolyte and that previously mentioned corresponds to the formula NiSn and contains approximately 65% tin and 35% nickel by Weight.

In order to determine the effects, if any, of electrolyte concentration and preplate thickness, I preplated a number of samples with tin-nickel alloy, some with a coating 1 micro-inch thick and some with a coating 2 microinches in thickness. The samples were then over-plated with tin in the amount of 1 lb. per base box and reflowed. Three concentrations of the electrolyte immediately abovementioned were used in the preplating step; that above set out, which may be called a full or high concentration, a /2 strength concentration and a /5 strength concentration. In the latter tWo cases each component of the electrolyte except the NaCl was reduced in the proportion indicated while the NaCl concentration remained unchanged. The reflow temperature was determined by sighting an optical pyrometer on the specimen while it was being heated, and is given as a range to indicate the accuracy of the measurement. The ATC values of the test specimens were determined, as well as those of controls which were not preplated. The results of this work are set out in Table I.

TABLE I ATC Value Reflow Preplate Specimen Electrolyte Concentemperature Thickness, tration Range, F. Microinches Control High Strength Strength It is immediately apparent from the table that the concentration of the preplate electrolyte has no significant effect on the corrosion resistance of the specimen. It is equally apparent that a preplate thickness of 1 microinch is entirely adequate, but that a thicker preplate does no harm. For economic reasons the amount of nickel should be kept to a minimum. Conceivably a preplate somewhat less than 1 micro-inch in thickness would be satisfactory, but it is quite difficult, even with a dilute electrolyte, to control plating conditions so as to deposit a uniform film of so light a weight.

I claim:

1. Electrotinplate of high corrosion resistance comprising a steel base, a fused unalloyed tin coating and an intermediate layer of a ternary alloy of iron, tin and nickel.

2. Electrotinplate of claim 1 in which the alloy layer is not less than about l 10 or greater than about 2 10 inches thick.

References Cited UNITED STATES PATENTS 2,085,543 6/1937 Opinger 29l96.6 X 2,176,066 10/1939 Domm. 2,274,963 3/1942 Hopper 29-1964 HYLAND BIZOT, Primary Examiner, 

1. ELECTROTINPLATE OF HIGH CORROSION REISTANCE COMPRISING A STEEL BASE, A FUSED UNALLOYED TIN COATING AND AN INTERMEDIATE LAYER OF A TERNARY ALLOY OF IRON, TIN AND NICKEL. 